It's not a slur itself though.

It was used by a schizophrenic man who developed his own operating system (TempleOS) and suffered paranoid delusions (I assume, maybe the feds were really doing some MKUltra stuff to him for some reason but on balance I suspect he was imagining it).

What he fully called them was "Glowing n-words", I'm pretty sure the less offensive "glowie" came about from others who heard about this and did find the idea of federal agents literally glowing kind of funny, glowing as in standing out.

So I wouldn't say it's offensive in this format. Because again it's not like Terry was some leader of white supremacy or something, he was a mentally ill man who suffered greatly in life but also did something pretty impressive and the whole idea of feds standing out in a visual way is pretty funny you have to admit.